no, you dont have to change the whole group. only the m732 front and rear mech are for 1989. the headset should be a 1 1/4" (m737) the other parts are correct, take a look:
http://mombat.org/Shimano.htm#1990
but I also put the m732 mechs on my 1990 merlin, its ok 8)
it gives 3 different...